LIGHTWEIGHT TWIST BEAM FINAL REPORT www.autosteel.org
LIGHTWEIGHT TWIST BEAM FINAL REPORT
Apr 05, 2023
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Microsoft Word - Lightweight Twist Beam Final Reportwww.autosteel.org
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Acknowledgements
OEM Project Team: Doug Howe Ford Motor Company Ranvir Singh Jalf Chrysler Group LLC William Pinch General Motors Company Cory Taulbert General Motors Company Multimatic Engineering Team: Nik Balaram Tudor Boiangiu Pardeep Dhillon Eric Gillund Bob Howell Scott Keefer Paul Saadetian Murray White
Steel Project Team: David Anderson Steel Market Development Institute Jon Fleck AK Steel Corporation Tom Wormold ArcelorMittal USA LLC Dean Kanelos Nucor Corporation Srinivasan Laxman Severstal North America Jon Powers Severstal North America Paul McKune ThyssenKrupp Steel USA, LLC Bart DePompolo United States Steel Corporation
Executive Summary
The objective of this project was to develop a lightweight steel proof-of-concept twist beam design that achieves a 15 to 25% mass reduction with equivalent structural and elasto-kinematic performance relative to the baseline design at a ≤ 10% cost premium. A current production original equipment manufacturer (OEM) twist beam assembly was used to establish the baseline for package, performance, mass and cost. Computer-aided engineering (CAE) structural optimization methods were used to determine the initial designs. Two designs were selected for further development and one design was subsequently selected as the best-performing and lightest alternative that met all typical performance criteria. An iterative optimization strategy was used to minimize the mass of each design, while meeting the specified strength, durability and elasto-kinematic requirements. The manufacturing cost was estimated for the preferred design relative to the baseline design for three production volumes. The results of the study indicate that the preferred U-Beam Design based on 22MnB5 tubular construction with DP780 and SPFH540 sheet achieves a 30.0% mass reduction relative to the baseline assembly, at a 12 to 15% premium in manufacturing cost. The S-Beam Design based on 22MnB5 sheet, DP780 tube and HSLA550 materials was predicted to have a 14.9% mass reduction relative to the baseline assembly. All designs were deemed manufacturable based on expert manufacturing assessment and relevant production application examples.
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Table of Contents
Executive Summary ..................................................................................................................... 3 Purpose .......................................................................................................................................... 9 Conclusions ................................................................................................................................... 9 Recommendations ........................................................................................................................ 9 Baseline Design .......................................................................................................................... 11 Design Targets ............................................................................................................................ 12
Structural Performance .......................................................................................................... 13 Mass .......................................................................................................................................... 14 Package ..................................................................................................................................... 16 Corrosion ................................................................................................................................. 16 Cost ........................................................................................................................................... 16
Development Process ................................................................................................................ 18 1. Concept Development ....................................................................................................... 18 2. Design Development .......................................................................................................... 19 3. Manufacturing and Corrosion .......................................................................................... 20 4. Cost Assessment ................................................................................................................. 20
Design – Package Effects ........................................................................................................... 21 Design Proposals ........................................................................................................................ 23
U-Beam Design ....................................................................................................................... 23 S-Beam Design ........................................................................................................................ 24
Performance ................................................................................................................................ 26 Materials ................................................................................................................................... 26 Material Modeling Considerations ...................................................................................... 28 Material Selection ................................................................................................................... 29 Durability ................................................................................................................................. 32 Extreme Loads ......................................................................................................................... 34 Performance Summary .......................................................................................................... 37
Mass ............................................................................................................................................. 38 Elasto-Kinematic Performance ................................................................................................. 40 Manufacturing ............................................................................................................................ 44
U-Beam Design ....................................................................................................................... 44 S-Beam Design ........................................................................................................................ 44
Corrosion ..................................................................................................................................... 46
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Cost Estimates ............................................................................................................................ 47 Assumptions ............................................................................................................................ 47
Material Costs ...................................................................................................................... 47 Design ................................................................................................................................... 47 Program ................................................................................................................................ 48 Variable Costs ...................................................................................................................... 48 Fixed Costs ........................................................................................................................... 48 Component Costs ................................................................................................................ 48
Cost Comparison .................................................................................................................... 48 References ................................................................................................................................... 51 Appendix 1: Kinematics and Compliance Plots .................................................................... 52
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List of Figures
Figure 1: Baseline OEM Twist Beam Assembly ....................................................................... 9 Figure 2: OEM Baseline Design ................................................................................................ 11 Figure 3: Twist Beam Design Targets ...................................................................................... 13 Figure 4: Structural Performance Targets ............................................................................... 14 Figure 5: Baseline twist beam Assembly Mass Summary .................................................... 15 Figure 6: Package Volume and Design Environment ........................................................... 16 Figure 7: Development Process Diagram ............................................................................... 18 Figure 8: Volume Topology Optimization ............................................................................. 19 Figure 9: Volume Topology Optimization Using an Extruded Constraint ....................... 19 Figure 10: Candidate Design Concepts ................................................................................... 20 Figure 11: Twist Beam Example with Outboard-Driven Hub Fasteners ........................... 21 Figure 12: Damper Attachment for OEM Baseline and U-Beam Designs ......................... 22 Figure 13: U-Beam Design Concept ........................................................................................ 24 Figure 14: S-Beam Design Concept ......................................................................................... 25 Figure 15: Twist Beam Finite Element Models ...................................................................... 26 Figure 16: Engineering Stress-Strain Curve Comparison .................................................... 27 Figure 17: Material Fatigue Property Reductions – HAZ .................................................... 28 Figure 18: HAZ – Durability Implementation ....................................................................... 29 Figure 19: HAZ – Strength Implementation .......................................................................... 29 Figure 20: U-Beam Design Material Selection and Gage...................................................... 30 Figure 21: S-Beam Design Material Selection and Gage....................................................... 31 Figure 22: OEM Baseline Design Material Selection and Gage ........................................... 31 Figure 23: Predicted Durability Life Comparison ................................................................. 32 Figure 24: Predicted OEM Baseline Design Durability Life ................................................ 33 Figure 25: Predicted U-Beam Design Durability Life ........................................................... 33 Figure 26: Predicted S-Beam Design Durability Life ............................................................ 34 Figure 27: Predicted Extreme Load Permanent Set Comparison ........................................ 35 Figure 28: Predicted OEM Baseline Extreme Load Plastic Strain ....................................... 35 Figure 29: Predicted U-Beam Design Extreme Load Plastic Strain .................................... 36 Figure 30: Predicted S-Beam Design Extreme Load Plastic Strain ..................................... 36 Figure 31: Twist Beam Assembly Mass Comparison ............................................................ 38 Figure 32: Twist Beam Structure Mass Comparison ............................................................. 39 Figure 33: K&C Results: Bounce – Bump Steer ...................................................................... 41
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Figure 34: K&C Results: Bounce – Bump Camber ................................................................ 41 Figure 35: K&C Results: Roll - Roll Steer ................................................................................ 42 Figure 36: K&C Results: Longitudinal Braking – Toe Stiffness ........................................... 42 Figure 37: K&C Results: Lat Parallel – Toe Stiffness ............................................................. 43 Figure 38: K&C Results: Align Opposed – Toe Stiffness ...................................................... 43 Figure 39: Stamping Formability for 22MnB5 Main Beam Structure ................................. 45 Figure 40: Relative Cost Comparison ...................................................................................... 49 Figure 41: Relative Cost Comparison Plot .............................................................................. 50
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List of Tables
Table 1: Detail OEM Baseline Twist Beam Assembly Mass Summary .............................. 15 Table 2: Steel Sheet Material Properties .................................................................................. 27 Table 3: Performance Summary ............................................................................................... 37 Table 4: Detail Mass Summary ................................................................................................. 39 Table 5: Assumed Material Costs ............................................................................................ 47
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Purpose
The objective of this project was to develop a lightweight steel proof-of-concept twist beam design that achieves a 15 to 25% mass reduction with equivalent structural and elasto-kinematic performance relative to the baseline design at a ≤ 10% cost premium. A current production OEM twist beam assembly was used to establish the baseline for package, performance, mass and cost.
Figure 1: Baseline OEM twist beam assembly
Conclusions
The results of the study support the following conclusions:
The U-Beam design is predicted to be 30.0% lighter than the OEM baseline design at a 12 to 15% cost premium at production volumes of 30,000 to 250,000 vehicles per year, respectively. The design is deemed production feasible based on expert manufacturing assessments.
The S-Beam design is predicted to have the best strength performance at a 14.9% mass reduction relative to the OEM baseline design. The design is deemed production feasible based on expert manufacturing assessments.
Recommendations
The twist beam designs are driven by durability and strength requirements at the component level and elasto-kinematic requirements at the vehicle level.
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Durability (Max Twist load case) and strength (Max Vertical load case) are the primary design drivers for both designs. CAE fatigue and strength modeling guidelines for the weld Heat Affected Zone (HAZ) have been developed based on Steel Market Development Institute (SMDI) recommendations. Currently, OEM best practices specify reduced material properties for all MIG welds and adjacent material in the weld HAZ to account for the effect of welding. Typically, the same reduced material properties are specified, regardless of the grade of steel. Some studies have shown that reduction in fatigue performance of advanced high-strength steel (AHSS) can be minimized by optimizing joint geometries [1]. Further study and development of robust high-volume welding practices and other advances in the area of sheet steel joining, especially with AHSS and ultra high-strength steel (UHSS) are recommended. Welding practices that result in improved HAZ properties could enable additional mass reduction by improving durability performance and thus more fully exploiting the benefits of high-strength materials in chassis components. Additionally, with the aggressive gage reductions enabled by the use of AHSS and UHSS, typical corrosion protection strategies may not be sufficient for these materials in chassis applications. Additional studies of the corrosion performance of these materials in welded assemblies, including pre- and post-assembly coatings, are recommended with the goal of developing definitive corrosion treatment strategies for chassis applications.
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Baseline Design
The baseline design, as chosen by the SMDI team, is depicted in Figure 2. The main structure is comprised of a tubular transverse beam and tubular trailing arms extending from the bushings to the damper mounts. Upper and lower reinforcements are added at the joints between the transverse and longitudinal members. The transverse beam features an inverted “V” in cross section, rotated from vertical orientation. Each damper mount is in single-shear via a threaded sleeve welded to the longitudinal tube. The spindle mounts are cantilevered above the trailing arm tubes and include reinforcement plates, machined for rear wheel static alignment.
Figure 2: OEM baseline design
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Design Targets
The overall project design targets are illustrated in the schematic shown in Figure 3. The objective was to develop a minimum mass design within the packaging constraints that met the structural and elasto-kinematic performance targets. Corrosion requirements are addressed by appropriate selection of material coatings, which typically do not add significant mass, but can increase cost. Mass and cost are the primary outputs of the study.
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Structural Performance
The specific structural performance requirements are summarized in the schematic shown in Figure 4. The fundamental design requirements are strength, durability and elasto-kinematic performance (as demonstrated by subsystem-level Kinematic and Compliance or K&C, performance). The strength requirements include (5) quasi-static extreme load cases in which the twist beam may not exhibit more than the allowable permanent set. The durability requirements include a total of (16) load cases that must be satisfied. Only the load cases that drive the designs will be discussed in this report. These include the top (3) extreme load cases and the top (3) durability load cases.
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Mass
A high-level breakdown of the OEM baseline twist beam assembly mass is shown in Figure 5. The complete assembly mass of 24.9 kg, including bushings, is used as the overall basis for comparison of the designs with respect to mass. A detailed component mass breakdown for the twist beam assembly is provided in Table 1.
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Table 1: Detail OEM baseline twist beam assembly mass summary
OEM Baseline Twist Beam Asy
Component kg % of total
Trailing Arm - Rr Axle 7.39 29.7%
Beam-Rear Axle 7.26 29.2%
Welding-Rr Axle 0.33 1.3%
Reinf-Beam Upr 0.48 1.9%
Reinf-Beam Lwr 0.44 1.8%
Bushings 2.19 8.8%
Mass per Asy
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Package
The overall design environment and resulting available package space is illustrated in Figure 6. The twist beam package space is defined by the fuel tank, the spare tire well, the rear floor pan, the tire envelope, the required clearances to these components and the required twist beam suspension travel.
Figure 6: Package volume and design environment
Corrosion
The project target for corrosion performance was based on typical OEM corrosion requirements. These requirements vary, but were assumed to require a minimum 10-year life in a highly-corrosive environment.
Cost
Recognizing the aggressive weight reduction targets enabled by the use of AHSS and UHSS, the project cost target was a ≤ 10% increase relative to the OEM baseline design. To assess cost, the manufacturing cost was estimated for the selected U-Beam proposal and compared to the baseline design cost. The project costing assumptions were:
Manufacturing cost for the twist beam assembly including the structure and bushings;
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Production volumes of 30,000, 100,000 and 250,000 vehicles per year; and
Program life of six years.
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Development Process
An iterative optimization strategy was used to minimize the mass of each design, while meeting the specified structural requirements. A schematic of the overall development strategy is shown in Figure 7. The key elements of the strategy are discussed in the following sections.
Figure 7: Development process diagram
1. Concept Development
Initial design concepts were developed based on size and shape optimization of the available design space shown in Figure 6. Stiffness and strength-based topology optimization methods were used to identify promising concepts using the optistruct solver [2]. Without manufacturing constraints, the optimization output was a truss structure with a distinct “U” shape in plain view. This result was interpreted into a concept “U-Beam” design as shown in Figure 8, named for its plan-view shape. Various draw constraints were also used to identify potential design concepts. The extruded constraint that resulted in a second initial concept is shown in Figure 9. This concept was termed the “S-Beam” since the cross-section developed into an “S” shape as additional optimization was performed.
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Figure 9: Volume topology optimization using an extruded constraint
2. Design Development
A total of two (2) candidate design concepts were identified in the concept development stage for further development. As indicated in Figure 10, these were the U-Beam and
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the S-Beam concepts. Various optimization strategies were utilized to meet each load requirement, while minimizing the overall mass. Shape optimization was used to develop the component geometry. Numerous additional design iterations were conducted to fine tune the material selection, thickness and local geometry to meet strength, durability and elasto-kinematic requirements. The elasto-kinematic requirements were cascaded into component twist beam stiffness requirements allowing for rapid early assessments in Abaqus before creation of flex bodies and full K&C assessments via Adams. The U-Beam design was later selected as the preferred alternative due to its superior structural, mass and elasto-kinematic performance. The S-Beam design details are also provided in this paper.
Figure 10: Candidate design concepts
3. Manufacturing and Corrosion
The manufacturing feasibility of each design was assessed at various stages of the development process. Additional design development was conducted to meet manufacturing feasibility requirements. Corrosion requirements including selection of coatings were considered as part of the manufacturing feasibility assessments.
4. Cost Assessment
The final step of the development process was to estimate the manufacturing cost for the selected U-Beam design and the OEM baseline design. Production costing methodologies were applied to estimate the manufacturing costs for the U-Beam design, and the costs were compared to the OEM baseline cost.
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Design – Package Effects
Two design changes with system-level effects were made to the candidate twist beams relative to the OEM baseline design: First, the hub mounting strategy will require attachment of the hub from the outboard rather than inboard direction (similar to the Honda Fit or Ford Fiesta designs as shown in Figure 11).
Figure 11: Twist beam example with outboard-driven hub fasteners
Second, the rear damper lower attachment was moved 40 mm outboard on both designs to facilitate a much improved load path to the beam structure. The move maintained the tire clearance envelope and resulted in a damper motion ratio change from 1.12 to 1.11. This is illustrated in Figure 12.
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Figure 12: Damper attachment for OEM baseline and U-Beam designs
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Design Proposals
The final U-Beam and S-Beam design proposals are shown in Figure 13 and Figure 14, respectively.
U-Beam Design
The U-Beam design utilizes UHSS and AHSS to enable aggressive gage and mass reductions. The U-Beam design features hot-formed tubular transverse and swept longitudinal members, all from 22MnB5 material with a constant 2.5 mm thickness. The transverse member has a closed inverted “U” cross section to provide the desired shear center location for roll steer performance. The roll steer can be tuned if required with this design by adding a rear-view sweep to the beam. The final design presented in this report has been tuned to achieve the OEM baseline roll steer with an unswept design for simplicity. The transverse member also features a fixed material gage but with a 20% increase in OD near vehicle centerline. This adds stiffness via section enlargement with minimum added mass. This increase in section is achieved either through the ACCRA® hot-forming process or by a purchased variable-diameter tube. The normally circular cross section of the longitudinal members is formed to a rectangular cross section at the hub mounts, facilitating integration of the hub attachment features without additional parts. A unique feature of this design is the addition of structural “bulkheads” to locally stabilize the beam assembly in the critical transition area from the lateral beam to the longitudinal arms. Trailing arms containing the bushings are simple inverted “U” profile stampings from DP780 material. All components are MIG-welded to form the assembly.
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S-Beam Design
The S-Beam design features a hot-stamped main beam and an associated hot-stamped lower reinforcement, all from 22MnB5 material. The stamped beam design provides the “S” cross section derived from optimization. The overall shape of the beam is also a “U” in the plan view, reflecting the optimization results consistently observed during development. The S-Beam also includes bulkheads to stabilize the lateral-to-longitudinal beam transition area. Trailing arms containing the bushings are closed-section tubular components from DP780 material, also designed to be compatible with the ACCRA® hot-forming process. All components are MIG-welded to form the assembly.
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Performance
Finite element (FE) analysis methods were used to predict the structural performance of each design. The FE model for the two new design concepts is shown in
Figure 15. As mentioned previously, an iterative optimization strategy was used to minimize the mass of each design while meeting the specified structural requirements. Optistruct [2], Abaqus / Standard [3] and nCode DesignLife [4] software products were used to optimize and assess the structural performance of designs. The final design material selections and structural performance are discussed in the following sections.
Figure 15: Twist Beam Finite Element Models
Materials
For the U-Beam and S-Beam designs, the material grade selection was primarily influenced by the durability and extreme load cases. Additional material selection criteria included formability, weldability, availability and cost. A table summarizing the Auto/Steel Partnership team recommended sheet materials is provided in Table 2. Engineering stress-strain curves for the sheet and forged materials utilized in this study are compared in Figure 16. The yield and ultimate tensile strengths are indicated for each material. Fatigue properties were obtained from [5].
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Material Modeling Considerations
Material processing considerations were taken into account in the finite element modeling. Specifically, the effects of welding-induced material property reduction in the Heat Affected Zone (HAZ) were included in the durability and strength load cases per the SMDI team’s recommendations as follows:
For the durability load cases, material fatigue property reductions of 20% in the weld HAZ were applied for all high-strength, advanced high-strength and ultra-high strength steel grades [6]. To achieve the reduced properties, the K’ parameter of the cyclic stress-strain amplitude curve was scaled by 0.8. The strain life curve was not modified. This is shown in Figure 17.
For strength load cases, material strength reductions of 20% in the HAZ zone were applied for UHSS grades with ultimate strengths greater than 800MPa. [6]. Shell welds were assigned properties corresponding to the lower-strength material of the two joining components. The shell weld thickness was determined by a weighted average between the two component thicknesses.
An example of the durability modeling…
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Acknowledgements
OEM Project Team: Doug Howe Ford Motor Company Ranvir Singh Jalf Chrysler Group LLC William Pinch General Motors Company Cory Taulbert General Motors Company Multimatic Engineering Team: Nik Balaram Tudor Boiangiu Pardeep Dhillon Eric Gillund Bob Howell Scott Keefer Paul Saadetian Murray White
Steel Project Team: David Anderson Steel Market Development Institute Jon Fleck AK Steel Corporation Tom Wormold ArcelorMittal USA LLC Dean Kanelos Nucor Corporation Srinivasan Laxman Severstal North America Jon Powers Severstal North America Paul McKune ThyssenKrupp Steel USA, LLC Bart DePompolo United States Steel Corporation
Executive Summary
The objective of this project was to develop a lightweight steel proof-of-concept twist beam design that achieves a 15 to 25% mass reduction with equivalent structural and elasto-kinematic performance relative to the baseline design at a ≤ 10% cost premium. A current production original equipment manufacturer (OEM) twist beam assembly was used to establish the baseline for package, performance, mass and cost. Computer-aided engineering (CAE) structural optimization methods were used to determine the initial designs. Two designs were selected for further development and one design was subsequently selected as the best-performing and lightest alternative that met all typical performance criteria. An iterative optimization strategy was used to minimize the mass of each design, while meeting the specified strength, durability and elasto-kinematic requirements. The manufacturing cost was estimated for the preferred design relative to the baseline design for three production volumes. The results of the study indicate that the preferred U-Beam Design based on 22MnB5 tubular construction with DP780 and SPFH540 sheet achieves a 30.0% mass reduction relative to the baseline assembly, at a 12 to 15% premium in manufacturing cost. The S-Beam Design based on 22MnB5 sheet, DP780 tube and HSLA550 materials was predicted to have a 14.9% mass reduction relative to the baseline assembly. All designs were deemed manufacturable based on expert manufacturing assessment and relevant production application examples.
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Table of Contents
Executive Summary ..................................................................................................................... 3 Purpose .......................................................................................................................................... 9 Conclusions ................................................................................................................................... 9 Recommendations ........................................................................................................................ 9 Baseline Design .......................................................................................................................... 11 Design Targets ............................................................................................................................ 12
Structural Performance .......................................................................................................... 13 Mass .......................................................................................................................................... 14 Package ..................................................................................................................................... 16 Corrosion ................................................................................................................................. 16 Cost ........................................................................................................................................... 16
Development Process ................................................................................................................ 18 1. Concept Development ....................................................................................................... 18 2. Design Development .......................................................................................................... 19 3. Manufacturing and Corrosion .......................................................................................... 20 4. Cost Assessment ................................................................................................................. 20
Design – Package Effects ........................................................................................................... 21 Design Proposals ........................................................................................................................ 23
U-Beam Design ....................................................................................................................... 23 S-Beam Design ........................................................................................................................ 24
Performance ................................................................................................................................ 26 Materials ................................................................................................................................... 26 Material Modeling Considerations ...................................................................................... 28 Material Selection ................................................................................................................... 29 Durability ................................................................................................................................. 32 Extreme Loads ......................................................................................................................... 34 Performance Summary .......................................................................................................... 37
Mass ............................................................................................................................................. 38 Elasto-Kinematic Performance ................................................................................................. 40 Manufacturing ............................................................................................................................ 44
U-Beam Design ....................................................................................................................... 44 S-Beam Design ........................................................................................................................ 44
Corrosion ..................................................................................................................................... 46
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Cost Estimates ............................................................................................................................ 47 Assumptions ............................................................................................................................ 47
Material Costs ...................................................................................................................... 47 Design ................................................................................................................................... 47 Program ................................................................................................................................ 48 Variable Costs ...................................................................................................................... 48 Fixed Costs ........................................................................................................................... 48 Component Costs ................................................................................................................ 48
Cost Comparison .................................................................................................................... 48 References ................................................................................................................................... 51 Appendix 1: Kinematics and Compliance Plots .................................................................... 52
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List of Figures
Figure 1: Baseline OEM Twist Beam Assembly ....................................................................... 9 Figure 2: OEM Baseline Design ................................................................................................ 11 Figure 3: Twist Beam Design Targets ...................................................................................... 13 Figure 4: Structural Performance Targets ............................................................................... 14 Figure 5: Baseline twist beam Assembly Mass Summary .................................................... 15 Figure 6: Package Volume and Design Environment ........................................................... 16 Figure 7: Development Process Diagram ............................................................................... 18 Figure 8: Volume Topology Optimization ............................................................................. 19 Figure 9: Volume Topology Optimization Using an Extruded Constraint ....................... 19 Figure 10: Candidate Design Concepts ................................................................................... 20 Figure 11: Twist Beam Example with Outboard-Driven Hub Fasteners ........................... 21 Figure 12: Damper Attachment for OEM Baseline and U-Beam Designs ......................... 22 Figure 13: U-Beam Design Concept ........................................................................................ 24 Figure 14: S-Beam Design Concept ......................................................................................... 25 Figure 15: Twist Beam Finite Element Models ...................................................................... 26 Figure 16: Engineering Stress-Strain Curve Comparison .................................................... 27 Figure 17: Material Fatigue Property Reductions – HAZ .................................................... 28 Figure 18: HAZ – Durability Implementation ....................................................................... 29 Figure 19: HAZ – Strength Implementation .......................................................................... 29 Figure 20: U-Beam Design Material Selection and Gage...................................................... 30 Figure 21: S-Beam Design Material Selection and Gage....................................................... 31 Figure 22: OEM Baseline Design Material Selection and Gage ........................................... 31 Figure 23: Predicted Durability Life Comparison ................................................................. 32 Figure 24: Predicted OEM Baseline Design Durability Life ................................................ 33 Figure 25: Predicted U-Beam Design Durability Life ........................................................... 33 Figure 26: Predicted S-Beam Design Durability Life ............................................................ 34 Figure 27: Predicted Extreme Load Permanent Set Comparison ........................................ 35 Figure 28: Predicted OEM Baseline Extreme Load Plastic Strain ....................................... 35 Figure 29: Predicted U-Beam Design Extreme Load Plastic Strain .................................... 36 Figure 30: Predicted S-Beam Design Extreme Load Plastic Strain ..................................... 36 Figure 31: Twist Beam Assembly Mass Comparison ............................................................ 38 Figure 32: Twist Beam Structure Mass Comparison ............................................................. 39 Figure 33: K&C Results: Bounce – Bump Steer ...................................................................... 41
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Figure 34: K&C Results: Bounce – Bump Camber ................................................................ 41 Figure 35: K&C Results: Roll - Roll Steer ................................................................................ 42 Figure 36: K&C Results: Longitudinal Braking – Toe Stiffness ........................................... 42 Figure 37: K&C Results: Lat Parallel – Toe Stiffness ............................................................. 43 Figure 38: K&C Results: Align Opposed – Toe Stiffness ...................................................... 43 Figure 39: Stamping Formability for 22MnB5 Main Beam Structure ................................. 45 Figure 40: Relative Cost Comparison ...................................................................................... 49 Figure 41: Relative Cost Comparison Plot .............................................................................. 50
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List of Tables
Table 1: Detail OEM Baseline Twist Beam Assembly Mass Summary .............................. 15 Table 2: Steel Sheet Material Properties .................................................................................. 27 Table 3: Performance Summary ............................................................................................... 37 Table 4: Detail Mass Summary ................................................................................................. 39 Table 5: Assumed Material Costs ............................................................................................ 47
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Purpose
The objective of this project was to develop a lightweight steel proof-of-concept twist beam design that achieves a 15 to 25% mass reduction with equivalent structural and elasto-kinematic performance relative to the baseline design at a ≤ 10% cost premium. A current production OEM twist beam assembly was used to establish the baseline for package, performance, mass and cost.
Figure 1: Baseline OEM twist beam assembly
Conclusions
The results of the study support the following conclusions:
The U-Beam design is predicted to be 30.0% lighter than the OEM baseline design at a 12 to 15% cost premium at production volumes of 30,000 to 250,000 vehicles per year, respectively. The design is deemed production feasible based on expert manufacturing assessments.
The S-Beam design is predicted to have the best strength performance at a 14.9% mass reduction relative to the OEM baseline design. The design is deemed production feasible based on expert manufacturing assessments.
Recommendations
The twist beam designs are driven by durability and strength requirements at the component level and elasto-kinematic requirements at the vehicle level.
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Durability (Max Twist load case) and strength (Max Vertical load case) are the primary design drivers for both designs. CAE fatigue and strength modeling guidelines for the weld Heat Affected Zone (HAZ) have been developed based on Steel Market Development Institute (SMDI) recommendations. Currently, OEM best practices specify reduced material properties for all MIG welds and adjacent material in the weld HAZ to account for the effect of welding. Typically, the same reduced material properties are specified, regardless of the grade of steel. Some studies have shown that reduction in fatigue performance of advanced high-strength steel (AHSS) can be minimized by optimizing joint geometries [1]. Further study and development of robust high-volume welding practices and other advances in the area of sheet steel joining, especially with AHSS and ultra high-strength steel (UHSS) are recommended. Welding practices that result in improved HAZ properties could enable additional mass reduction by improving durability performance and thus more fully exploiting the benefits of high-strength materials in chassis components. Additionally, with the aggressive gage reductions enabled by the use of AHSS and UHSS, typical corrosion protection strategies may not be sufficient for these materials in chassis applications. Additional studies of the corrosion performance of these materials in welded assemblies, including pre- and post-assembly coatings, are recommended with the goal of developing definitive corrosion treatment strategies for chassis applications.
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Baseline Design
The baseline design, as chosen by the SMDI team, is depicted in Figure 2. The main structure is comprised of a tubular transverse beam and tubular trailing arms extending from the bushings to the damper mounts. Upper and lower reinforcements are added at the joints between the transverse and longitudinal members. The transverse beam features an inverted “V” in cross section, rotated from vertical orientation. Each damper mount is in single-shear via a threaded sleeve welded to the longitudinal tube. The spindle mounts are cantilevered above the trailing arm tubes and include reinforcement plates, machined for rear wheel static alignment.
Figure 2: OEM baseline design
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Design Targets
The overall project design targets are illustrated in the schematic shown in Figure 3. The objective was to develop a minimum mass design within the packaging constraints that met the structural and elasto-kinematic performance targets. Corrosion requirements are addressed by appropriate selection of material coatings, which typically do not add significant mass, but can increase cost. Mass and cost are the primary outputs of the study.
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Structural Performance
The specific structural performance requirements are summarized in the schematic shown in Figure 4. The fundamental design requirements are strength, durability and elasto-kinematic performance (as demonstrated by subsystem-level Kinematic and Compliance or K&C, performance). The strength requirements include (5) quasi-static extreme load cases in which the twist beam may not exhibit more than the allowable permanent set. The durability requirements include a total of (16) load cases that must be satisfied. Only the load cases that drive the designs will be discussed in this report. These include the top (3) extreme load cases and the top (3) durability load cases.
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Mass
A high-level breakdown of the OEM baseline twist beam assembly mass is shown in Figure 5. The complete assembly mass of 24.9 kg, including bushings, is used as the overall basis for comparison of the designs with respect to mass. A detailed component mass breakdown for the twist beam assembly is provided in Table 1.
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Table 1: Detail OEM baseline twist beam assembly mass summary
OEM Baseline Twist Beam Asy
Component kg % of total
Trailing Arm - Rr Axle 7.39 29.7%
Beam-Rear Axle 7.26 29.2%
Welding-Rr Axle 0.33 1.3%
Reinf-Beam Upr 0.48 1.9%
Reinf-Beam Lwr 0.44 1.8%
Bushings 2.19 8.8%
Mass per Asy
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Package
The overall design environment and resulting available package space is illustrated in Figure 6. The twist beam package space is defined by the fuel tank, the spare tire well, the rear floor pan, the tire envelope, the required clearances to these components and the required twist beam suspension travel.
Figure 6: Package volume and design environment
Corrosion
The project target for corrosion performance was based on typical OEM corrosion requirements. These requirements vary, but were assumed to require a minimum 10-year life in a highly-corrosive environment.
Cost
Recognizing the aggressive weight reduction targets enabled by the use of AHSS and UHSS, the project cost target was a ≤ 10% increase relative to the OEM baseline design. To assess cost, the manufacturing cost was estimated for the selected U-Beam proposal and compared to the baseline design cost. The project costing assumptions were:
Manufacturing cost for the twist beam assembly including the structure and bushings;
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Production volumes of 30,000, 100,000 and 250,000 vehicles per year; and
Program life of six years.
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Development Process
An iterative optimization strategy was used to minimize the mass of each design, while meeting the specified structural requirements. A schematic of the overall development strategy is shown in Figure 7. The key elements of the strategy are discussed in the following sections.
Figure 7: Development process diagram
1. Concept Development
Initial design concepts were developed based on size and shape optimization of the available design space shown in Figure 6. Stiffness and strength-based topology optimization methods were used to identify promising concepts using the optistruct solver [2]. Without manufacturing constraints, the optimization output was a truss structure with a distinct “U” shape in plain view. This result was interpreted into a concept “U-Beam” design as shown in Figure 8, named for its plan-view shape. Various draw constraints were also used to identify potential design concepts. The extruded constraint that resulted in a second initial concept is shown in Figure 9. This concept was termed the “S-Beam” since the cross-section developed into an “S” shape as additional optimization was performed.
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Figure 9: Volume topology optimization using an extruded constraint
2. Design Development
A total of two (2) candidate design concepts were identified in the concept development stage for further development. As indicated in Figure 10, these were the U-Beam and
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the S-Beam concepts. Various optimization strategies were utilized to meet each load requirement, while minimizing the overall mass. Shape optimization was used to develop the component geometry. Numerous additional design iterations were conducted to fine tune the material selection, thickness and local geometry to meet strength, durability and elasto-kinematic requirements. The elasto-kinematic requirements were cascaded into component twist beam stiffness requirements allowing for rapid early assessments in Abaqus before creation of flex bodies and full K&C assessments via Adams. The U-Beam design was later selected as the preferred alternative due to its superior structural, mass and elasto-kinematic performance. The S-Beam design details are also provided in this paper.
Figure 10: Candidate design concepts
3. Manufacturing and Corrosion
The manufacturing feasibility of each design was assessed at various stages of the development process. Additional design development was conducted to meet manufacturing feasibility requirements. Corrosion requirements including selection of coatings were considered as part of the manufacturing feasibility assessments.
4. Cost Assessment
The final step of the development process was to estimate the manufacturing cost for the selected U-Beam design and the OEM baseline design. Production costing methodologies were applied to estimate the manufacturing costs for the U-Beam design, and the costs were compared to the OEM baseline cost.
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Design – Package Effects
Two design changes with system-level effects were made to the candidate twist beams relative to the OEM baseline design: First, the hub mounting strategy will require attachment of the hub from the outboard rather than inboard direction (similar to the Honda Fit or Ford Fiesta designs as shown in Figure 11).
Figure 11: Twist beam example with outboard-driven hub fasteners
Second, the rear damper lower attachment was moved 40 mm outboard on both designs to facilitate a much improved load path to the beam structure. The move maintained the tire clearance envelope and resulted in a damper motion ratio change from 1.12 to 1.11. This is illustrated in Figure 12.
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Figure 12: Damper attachment for OEM baseline and U-Beam designs
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Design Proposals
The final U-Beam and S-Beam design proposals are shown in Figure 13 and Figure 14, respectively.
U-Beam Design
The U-Beam design utilizes UHSS and AHSS to enable aggressive gage and mass reductions. The U-Beam design features hot-formed tubular transverse and swept longitudinal members, all from 22MnB5 material with a constant 2.5 mm thickness. The transverse member has a closed inverted “U” cross section to provide the desired shear center location for roll steer performance. The roll steer can be tuned if required with this design by adding a rear-view sweep to the beam. The final design presented in this report has been tuned to achieve the OEM baseline roll steer with an unswept design for simplicity. The transverse member also features a fixed material gage but with a 20% increase in OD near vehicle centerline. This adds stiffness via section enlargement with minimum added mass. This increase in section is achieved either through the ACCRA® hot-forming process or by a purchased variable-diameter tube. The normally circular cross section of the longitudinal members is formed to a rectangular cross section at the hub mounts, facilitating integration of the hub attachment features without additional parts. A unique feature of this design is the addition of structural “bulkheads” to locally stabilize the beam assembly in the critical transition area from the lateral beam to the longitudinal arms. Trailing arms containing the bushings are simple inverted “U” profile stampings from DP780 material. All components are MIG-welded to form the assembly.
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S-Beam Design
The S-Beam design features a hot-stamped main beam and an associated hot-stamped lower reinforcement, all from 22MnB5 material. The stamped beam design provides the “S” cross section derived from optimization. The overall shape of the beam is also a “U” in the plan view, reflecting the optimization results consistently observed during development. The S-Beam also includes bulkheads to stabilize the lateral-to-longitudinal beam transition area. Trailing arms containing the bushings are closed-section tubular components from DP780 material, also designed to be compatible with the ACCRA® hot-forming process. All components are MIG-welded to form the assembly.
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Performance
Finite element (FE) analysis methods were used to predict the structural performance of each design. The FE model for the two new design concepts is shown in
Figure 15. As mentioned previously, an iterative optimization strategy was used to minimize the mass of each design while meeting the specified structural requirements. Optistruct [2], Abaqus / Standard [3] and nCode DesignLife [4] software products were used to optimize and assess the structural performance of designs. The final design material selections and structural performance are discussed in the following sections.
Figure 15: Twist Beam Finite Element Models
Materials
For the U-Beam and S-Beam designs, the material grade selection was primarily influenced by the durability and extreme load cases. Additional material selection criteria included formability, weldability, availability and cost. A table summarizing the Auto/Steel Partnership team recommended sheet materials is provided in Table 2. Engineering stress-strain curves for the sheet and forged materials utilized in this study are compared in Figure 16. The yield and ultimate tensile strengths are indicated for each material. Fatigue properties were obtained from [5].
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Material Modeling Considerations
Material processing considerations were taken into account in the finite element modeling. Specifically, the effects of welding-induced material property reduction in the Heat Affected Zone (HAZ) were included in the durability and strength load cases per the SMDI team’s recommendations as follows:
For the durability load cases, material fatigue property reductions of 20% in the weld HAZ were applied for all high-strength, advanced high-strength and ultra-high strength steel grades [6]. To achieve the reduced properties, the K’ parameter of the cyclic stress-strain amplitude curve was scaled by 0.8. The strain life curve was not modified. This is shown in Figure 17.
For strength load cases, material strength reductions of 20% in the HAZ zone were applied for UHSS grades with ultimate strengths greater than 800MPa. [6]. Shell welds were assigned properties corresponding to the lower-strength material of the two joining components. The shell weld thickness was determined by a weighted average between the two component thicknesses.
An example of the durability modeling…
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